1. Field
The disclosed embodiments relate to interface systems for reducing particle contamination of substrates during processing.
2. Brief Description of Related Developments
There is a desire in the semiconductor industry to reduce wafer cycle time through the FAB and reduce the amount of work in progress (WIP) as well as to improve wafer safety. Conventionally, the carrier to load port physical interface is a multi-step process involving up to six mechanisms to perform the carrier load and unload operation. In this environment, the load port cycle time ranges for example from 12-18 seconds depending on the manufacturer and can reach 2 million cycles over a 7 year life in extreme applications. A lot size of 25 wafers per conventional carrier is used to optimize tool utilization and minimize the effect of tool setup and wafer handling overhead. Conventional semiconductor manufacturing has been generally focussed on high volume with a low mix of product types flowing through the production line. In contrast, the practices of the manufacturing environment has tended to migrate so that it consists of many product types of both high and low volume. Essentially, changes in the semiconductor business model are driving fab managers to minimize inventories and reduce manufacturing cycle times. The later is heavily influenced by the lot size in the wafer carrier. Some suggest that a lot size smaller than 13 wafers is a point at which significant gains in cycle time can be realized. One end of this approach is to drive the lot size for example to a single wafer. Although a single wafer may be theoretically ideal, the current state of process tool architecture is not compatible with the level of recipe changes related thereto and thus drives up tool setup time. The length of setup time on some tools can be equal to or greater than the processing time of the single wafer negating the original intentions. In addition, due to the complexity in characterizing advanced process tools, it is desired that some number of test or qualification wafers confirm that the process is operating within specification. These non-product wafers may be used and handled in conjunction with a single wafer strategy.
A smaller, multi-wafer lot size may be effectively employed to support the single wafer strategy. However, as may be realized, variance in the lot size of the carrier has a commensurate impact on the load port design. Specifically, the cycle time of the load port may be generally linearly proportional to the lot size. For example, to avoid limiting the process tool throughput a 12 second cycle time on a 25 wafer lot may use a 2.4 second cycle time on a 5 wafer lot. Recalculation of the load port life with a reduced cycle time results in 10 million cycles over a 7 year life for the same steady state throughput. A further aspect of a load port which can open or close the carrier in ⅕ of the time it must possess inherent reliability; otherwise, the mean cycle between failures (MCBF) of the load port will negatively impact the tool level MCBF.
On the other hand, the impact to the carrier from a reduction in lot size and cycle time is two-fold. First, the lot size reduction impacts the time to open or close the carrier on the load port. Second, manufacturing cycle time impacts the number of desired open/close cycles of the carrier. A simple calculation can approximate the total cycles of a carrier based upon the number of mask layers, process steps per layer and the days per mask layer. Currently 27 mask layers with 32 process steps each are typical. The number of days per mask layer varies depending on the device and manufacturer but a reasonable estimate is 1.5 days per masks layer. For the purposes of the example calculation it may be assumed that the carrier may be loaded onto a different tool for each process step (a conservative assumption).Process Steps÷Days per Mask Layer=Cyclescarrier/Day32÷1.5=21.33 Cyclescarrier/Day
Taken to the extreme, device manufacturers have suggested that it is highly desired that the days per mask layer be reduced to 1-0.7 days to achieve optimal productivity and that future-devices may employ up to 45 mask layers. Inserting the forecasted changes into our previous example calculation we compute the following new values for carrier cycle time. The number of process steps per mask layer is assumed to be unchanged.Process Steps÷Days per Mask Layer=Cyclescarrier/Day32÷0.7=45.7 Cyclescarrier/Day
Based on the previous exemplary calculations we can derive the cycles over a seven year life of the carrier to be between 54,498 and 116,764. In other words a carrier may be subjected to open and close cycle every 31.5 minutes. Conventional load ports, carriers and the interface therebetween cannot satisfy the anticipated operational parameters. Associated with the desire for more robust carrier and carrier to tool interface (for example with respect to ability to withstand substantially higher (e.g. ×2-×10) cycles and hence provide higher cleanliness within the carrier and across the interface) is the desire to simplify and speed the system that gets the substrates to the different process modules, that carry out the various process steps, bending what is achievable with conventional load ports and carriers.